The present invention relates to polyphase dc motors in general, but has particular application with three phase dc motors of the brushless type which are used for rotating data media. Such motors are often found in computer related applications, including hard disk drives, CD ROM drives, floppy disks, and the like. In computer applications, such motors are becoming more popular due to their reliability, low weight, and accuracy.
Three phase motors typically are thought of as having a stator with three coils arranged in the shape of a "Y" configuration, with the common connection at the center of the "Y" known as the stator center tap. Note, however, that often more than three coils are used. In any instance, the stator coils may be analyzed in terms of the three "Y" connected coils. To operate the motor, current is passed through one or more of the coils, and switched (referred to as "commuted" or the act of "commutation") at desired times to effect changing magnetic fields, thereby causing the magnetic rotor to rotate with respect to the stator coils.
The present invention applies to such commutating configurations which operate in at least two modes, namely, a bipolar mode or a unipolar mode. In both modes, different current paths through the coils are created by selectively turning on and off power to the coils in a sequence. In the bipolar mode, for each path in the sequence, only two of the three coils conduct while the third coil floats. Further, the sequence is defined so that when the floating coil is switched into the current path, current flows through the newly added coil in the same direction which it flowed through the coil which is now removed from the path. In this manner, six sequences are defined for each electrical cycle of a three phase motor. In the unipolar mode of operation, only one coil conducts current while the other two coils float. The center tap may be coupled to the voltage source or grounded.
The contrast of the bipolar and unipolar (or "uni-coil") modes are detailed in U.S. patent application Ser. No. 08/123,347, entitled "Method And Apparatus For Starting A Sensorless Polyphase DC Motor In Dual Coil Mode And Switching To Single Coil Mode At Speed," filed Sep. 17, 1993, having inventor Scott Cameron, and this Application is hereby fully incorporated herein by reference. As an overview, however, a brief description follows demonstrating the effect of each mode and the benefits of switching from bipolar mode to unipolar mode once a sufficient rotor speed is achieved.
The maximum achievable motor speed in the bipolar mode is related to the maximum torque that the coil arrangement can generate, and the maximum torque is related to the maximum current available for passing through both coils. The goal is to achieve a maximum torque with a fixed current but, as always, the current is determined by the available power. However, as known in the art, a spinning rotor induces a back emf in the stator coils and, as described below, the back emf effectively reduces the amount of source voltage and, therefore, reduces the amount of current available to pass through the coils.
The back emf effect is understood in considering the operation of the motor from start-up to a satisfactory speed. Before the rotor begins to spin, there is no back emf. In contrast, at increasing rotor speeds, the back emf increases and, for a given voltage, correspondingly limits the amount of current that can be applied to the coils (again, limiting the torque because of the limit on current). Therefore, in order to reduce the back emf (and its current limiting effect), the stator is switched to the unipolar mode, thereby halving the number of flux linkages or coil turns in the current path (i.e., by reducing the number of coils from two to one). By reducing the coil turns, the back emf is reduced, thereby freeing more current to drive the single stator coil in the unipolar mode. As detailed in the above-incorporated patent application, the additional current then further increases the speed of the motor from its point at which back emf created a limitation to torque in the bipolar mode.
From the above, one skilled in the art will recognize that the combined use of both the bipolar and unipolar mode increases efficiencies and operability as described and incorporated above. The present invention further improves product performance and reduces otherwise existing problems by controlling the timing of the switch between the bipolar mode and the unipolar mode.
It is therefore an object of the present invention to provide a method and apparatus for switching a motor between bipolar and unipolar mode at a controlled time.
It is a further object of the present invention to provide such a method and apparatus for switching a motor between bipolar and unipolar mode at or near the midpoint between zero crossings of the back emf of the floating coil.
It is a further object of the present invention to provide such a method and apparatus for switching a motor between bipolar and unipolar mode at or near the point of commutation.
It is a further object of the present invention to provide such a method and apparatus for switching a motor between bipolar and unipolar mode with minimal affect on spindle sequencing.
It is a further object of the present invention to provide such a method and apparatus for switching a motor between bipolar and unipolar mode with minimal affect on motor torque.
It is a further object of the present invention to provide such a method and apparatus for switching a motor between bipolar and unipolar mode at or near the point of high-to-high commutation.
Still other objects and advantages of the present invention will become apparent to those of ordinary skill in the art having references to the following specification together with its drawings.